Siemens SIMATIC FM 353 Manual page 297

Infinite loop M18
M18 is always output as the last M function in the block.
Two cases are distinguished:
S M function M18 is output like any other M function. Only after the block has
been processed all the way to the end (including M18) does the axis skip back
to the start of the program.
S If M function M18 is programmed alone in the last block of a traversing pro-
gram, the M function is not output, and the axis immediately skips back to the
start of the program.
Change signal programmable as digital output M97, M98
If M97 or M98 is programmed in a block, the M function output proceeds via the
digital outputs as defined in machine data item MD35, in the same way as the
checkback signals.
Tool offset number D
Twenty tool offset numbers (D1 − D20) are available. D0 in conjunction with G43 or
G44 causes the tool offset to be switched off. The offset values must previously
have been loaded to the module. Nonstandard offset values have a value 0.
Subprogram call P, L
A block with a subprogram call (P is the "number of runs", L is the "program num-
ber") cannot contain any further information.
A maximum of 19 subroutines can be called up in one program. Nesting is not
possible.
P = Number of subprogram runs
FM 353 Stepper Drive Positioning Module
6ES7 353-1AH01-8BG0
Name
Writing Traversing Programs
Lower input limit Upper input limit
1 250
10-15